#include <stdio.h>
#define PI 3.14

int main() {
    int l;
    scanf("%d", &l);
    double square_area = l * l;
    double circle_area = PI * (l / 2.0) * (l / 2.0);
    double result = square_area - circle_area;
    printf("%.2f\n", result);
    return 0;
}
